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Diffstat (limited to 'doc/src')
| -rw-r--r-- | doc/src/sgml/config.sgml | 38 |
1 files changed, 16 insertions, 22 deletions
diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml index 7ece94474d9..8c82b39a89d 100644 --- a/doc/src/sgml/config.sgml +++ b/doc/src/sgml/config.sgml @@ -2574,39 +2574,33 @@ include_dir 'conf.d' </term> <listitem> <para> - Sets the number of concurrent disk I/O operations that + Sets the number of concurrent storage I/O operations that <productname>PostgreSQL</productname> expects can be executed simultaneously. Raising this value will increase the number of I/O - operations that any individual <productname>PostgreSQL</productname> session - attempts to initiate in parallel. The allowed range is 1 to 1000, - or zero to disable issuance of asynchronous I/O requests. Currently, - this setting only affects bitmap heap scans. + operations that any individual <productname>PostgreSQL</productname> + session attempts to initiate in parallel. The allowed range is + <literal>1</literal> to <literal>1000</literal>, or + <literal>0</literal> to disable issuance of asynchronous I/O requests. + The default is <literal>16</literal> on supported systems, otherwise + <literal>0</literal>. </para> <para> - For magnetic drives, a good starting point for this setting is the - number of separate - drives comprising a RAID 0 stripe or RAID 1 mirror being used for the - database. (For RAID 5 the parity drive should not be counted.) - However, if the database is often busy with multiple queries issued in - concurrent sessions, lower values may be sufficient to keep the disk - array busy. A value higher than needed to keep the disks busy will - only result in extra CPU overhead. - SSDs and other memory-based storage can often process many - concurrent requests, so the best value might be in the hundreds. + Higher values will have the most impact on higher latency storage + where queries otherwise experience noticeable I/O stalls and on + devices with high IOPs. Unnecessarily high values may increase I/O + latency for all queries on the system </para> <para> - Asynchronous I/O requires that the operating system supports issuing - read-ahead advice. If there is no operating system support then - setting this parameter to anything but zero will result in an error. + On systems without prefetch advice support, attempting to configure + any value other than <literal>0</literal> will error out. </para> <para> - The default is 1 on supported systems, otherwise 0. This value can - be overridden for tables in a particular tablespace by setting the - tablespace parameter of the same name (see - <xref linkend="sql-altertablespace"/>). + This value can be overridden for tables in a particular tablespace by + setting the tablespace parameter of the same name (see <xref + linkend="sql-altertablespace"/>). </para> </listitem> </varlistentry> |